Gaming

Kirby. Any Kirby.

He's still pretty young to be a player 2, but he can still be your assistant!

Mario galaxy 1 and 2 let a second player play as luma, who can paralyze enemies, help mario jump and gather star shards.

It's also a very soothing and (for an adult) easy game. I have fond memories of the music and all the various galaxies, if you can give it a try!

Nintendo is hard to beat in the field of family friendly games. Captain Toad is probably the most child friendly game of them all. Mario Odyssey should probably be fun as well. Could really motivate him to learn counting as he looks for the collectibles.

My son is 6yo so I have some experience. Usually, chill games like Little Big Planets, Astro Bot, or even Marios catches his attention(he loved goomba chasing mario around), as long as you are fine that they won't "progress" the game at 3yo, then you are fine and just follow the age rating to select your game.

Another one is racing game, might be harder for them to play but my son loves watching me play rocket league, and I tried a couple racing games he was interested as well. For games that more or less played like roller-coaster rides usually attract him longer.

Then, another really good one is 3rd person open world games. As you can try communicate with him and have lots of conversation as well even when he can't control the character yet. There should be children safe version, for me when I played AC:Odyssey, he just like me pilot the ship going around or go diving and get chased by the sharks. (I basically avoid fighting people when he is watching, and just doing puzzles, cave exploration, etc.)

Feel free to try a bunch of different games and see which one sticks. :) Remember don't play games that frustrate your easily cause kids can tell if you are stressed.

Any of the LEGO games are a great choice starting around ages 3 or 4. They use button picture prompts, and often obstacles can be overcome by the parent while the little one jumps around smashing things.

Re-enforces moving around in a 3d space, and gets good familiarity with button locations doing different things (while really only needing jump and punch for most things).

After a while with LEGO, started getting into games like Minecraft Dungeons - again parent can do most of the work while the little one tags along for the ride.

Was around age 5/6 that my son was able to pick up Minecraft - nothing sophisticated and again I did most of the building, but allowed him to contribute more than he had previous games, and strongly builds spatial awareness skills. Pokemon also became accessible as reading skills were good enough to understand what was on the screen.

Minecraft obviously. Very conceptually easy. Little to no reading needed to grasp basics. Very deep and rewarding for adults as well (redstone logic, exploring, creative building)

Rocket League maybe once he's able to grasp driving more.

I think you should check out "Everything". Very enjoyable and easy, it will even play itself if you let the controller go.

Check out Spiritfarer. Very relaxing game where you sail around on a boat guiding spirits on their path to afterlife. Sounds grim but it is very colourful and everything is only implied.

You upgrade your boat by visiting islands and collecting resources. The cool thing about it is that the player 2 controls a cat that can jump around and helps with stuff. There is fishing, farming, cooking and crafting.

The characters and world is very childlike and there are many sparkly things to collect for your kid once he gets used to the controls.

You can turn on assistance on MK8D. You can turn on auto accelerate and assisted steering. It is in the pause menu.

Kirby is a really good series for kids. The Switch had the retros as well that might be easier (2d instead of 3d).

Edit. Mario Odyssey has a assistance mode as well.

Edit 2: just thought of another with an easy mode, Yoshi on the Switch.